Page 183 - Microtectonics

172 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins
will be clear, however, that this will not apply in the case neous shortening axis (one of the ISA). In coaxial pro-
of complex vein shapes. This means that curved fibres gressive deformation, this has no implications for the
may track the opening direction of the vein (Fig. 6.7), shape of the vein, but in non-coaxial flow the older cen-
but not necessarily the instantaneous extension direc- tral part of a vein will rotate just like any material line in
tion in the bulk rock (Sect. 6.3). It is therefore necessary the bulk flow (Fig. 5.39) while the tension gash still propa-
to be careful when using fibrous aggregates for kinematic gates outward along the instantaneous shortening axis.
analysis; the nature of the fibre growth process should The older part of the vein is deformed in this process;
first be established. antitaxial calcite veins commonly show deformation twins
We limit the treatment of veins and strain shadows in in the oldest parts of the calcite fibres that lie in the cen-
this chapter mainly to pure shear and simple shear pro- tre of the vein (Burkhard 1993) and quartz veins may show
gressive deformation histories. Obviously, other flow re- kink bands, shear bands, microcracks and small cavities
gimes and more complex histories involving volume (Nishikawa and Takeshita 2000; van Daalen et al. 1999;
change are also possible. However, the end-member situ- Smith 2005). The younger, outer parts have rotated less,
ations will in most cases allow assessment of at least shear and this explains the curved shape of tension gash veins
sense, and permit establishment of the type of deviation formed in non-coaxial progressive deformation (Fig. 6.17).
from the ideal model. The ‘en-echelon’ arrangement of tension gashes in many
shear zones occurs because tension gashes start to de-
6.2.4 velop where small cracks are formed at regular intervals
Veins in Non-Coaxial Progressive Deformation along the central part of a shear zone. Because of their
simple development mechanism, the shape of tension
In brittle fault zones and some ductile shear zones, tension gashes is a reliable shear-sense indicator; S-shaped and
gashes develop in sets of veins that are arranged en-ech- Z-shaped gashes indicate sinistral and dextral shear sense
elon (“Echelon tension gashes”: Fig. 6.16a, ×Video 6.16; respectively (Fig. 6.16a, ×Video 6.16; Choukroune and
Olson and Pollard 1991; Peacock and Sanderson 1995). If Seguret 1968; Ramsay and Graham 1970; Beach 1975;
such echelon tension gashes lie close together, the strips Gamond 1983). In some cases, two generations of tension
of wall rock separating them are known as bridges gashes are found, the younger one transecting the centre
(Fig. 6.16b). Large tension gashes in shear zones usually of the older one parallel to its tips (Fig. 6.16a). Such
have a characteristic, curved geometry (Fig. 6.16a). In both ‘secondary’ tension gashes are thought to form in response
coaxial and non-coaxial flow, tension gashes open in a to rotation of the central segment of the older gash into the
direction approximately parallel to the direction of maxi- extensional field of the flow (Sect. 5.6.16; Fig. 5.39). Simi-
mum instantaneous extension. If the veins form during larly, stylolites (Box 4.3) are sometimes found at a high
progressive deformation, they may both become wider angle to developing tension gashes. Notice that a folia-
and propagate laterally outward parallel to the instanta- tion in a shear zone will form oblique to the gashes, and

Fig. 6.16.
a Schematic diagram of tension
gashes in en-echelon arrange-
ment in a shear zone. A second
set of tension gashes develops
in the centre of the older set.
The sense of curvature for ten-
sion gashes and foliation in a
shear zone is similar. Stylolites
may develop cutting tension
gashes or the foliation. b Tension
gashes commonly pass into con-
tinuous veins laterally through
feather veins. Feather veins prob-
ably form when a continuous
vein grows over a series of en
echelon tension gashes. If ten-
sion gashes are close together,
the interfering strips of rock
are known as bridges